Maintaining indoor air quality (IAQ) involves monitoring and managing pollutants, temperature and humidity using a ventilation and air conditioning system.
The sources of indoor air pollutants may be biological, for example human respiration, which releases pathogenic particles such as viruses and bacteria. Mould can also proliferate in environments with high relative humidity, releasing spores. Other sources of pollutants are linked to human activities like cooking food and burning wood. Finally, there are sources of pollutants deriving from the materials used to construct the building and its interiors, which release volatile organic compounds (VOCs) into the environment.

The main sources of outdoor pollutants are vehicular traffic, industries, heating systems, agriculture, and natural emissions, such as desert dust and pollen.

The substantial difference between indoor and outdoor pollutants involves the substances released by human respiration. In outdoor environments, these particles are quickly diluted in the atmosphere, but in closed indoor spaces, they can reach high concentrations and be a source of airborne diseases (e.g. COVID).

Different indoor environments can be occupied in different ways. Classrooms tend to have a high density of occupants, who spend relatively little time inside. In offices, occupant density usually is lower, however the time spent inside is higher, spanning the entire working day. In healthcare facilities, the situation depends on the department, which may have different needs in terms of the type of patients and the treatments provided.

Except for some sources of pollution that are typical of the specific environment, such as chalk in schools and printers in offices, a significant portion of the pollutants present are due to the respiration of occupants. Humans emit CO₂ while breathing, which is not harmful in low concentrations. CO₂ concentration is technically easy and economical to measure; for this reason, it can be considered the reference parameter for setting the operating conditions of the ventilation and air conditioning systems. Diluting the CO₂ concentration means dilution of the total pollutant load.

Ventilation and air conditioning systems allow air from outside with a low CO₂ concentration (450 ppm) to be delivered into buildings. The air is then conditioned to guarantee the ideal temperature and relative humidity. The buildings can be equipped with temperature, relative humidity, and CO₂ sensors. These sensors send the parameters to the ventilation and air conditioning system’s electronic controller, which manages the incoming and exhaust air flow rates. The control algorithms synchronise the various components of the system, such as dampers, heat exchangers, and humidifiers, maximising energy savings.

Monitoring and maintaining the right relative humidity (RH) is important for human health. In colder times of the year, indoor relative humidity is often very low (<25%), and this phenomenon could be amplified by a ventilation and air conditioning system without a humidifier.

RH values lower than the optimal range of 40–60% lead to dryness of the mucous membranes in the respiratory system. This weakens the human body’s defences and makes us more vulnerable to viruses and bacteria. Furthermore, dry air causes sensations of discomfort, such as dry skin and eye irritation.

The air handling unit (AHU), the heart of the ventilation and air conditioning system, when correctly sized, can simultaneously keep temperature, relative humidity and CO₂ at the desired values, guaranteeing good IAQ and a healthy environment for all building occupants.